Just wondering if the Dynaco would take a 5R4 or 5U4 rectifier in place of the stock 5AR4. I have plenty of the 5R4's and 5U4's but no extra 5AR4's. I thought about SS rectification but then my B+ exceeds the rating of the power supply caps. It would also push the B+ above 300V on the plates of my 12AT7's in my non Dynaco front end and driver board.

I think the data sheets would make it clear if a) they fit physically in the same socket and b) if they can support the same or larger voltage and current. Or is there some other magic factor I missed??

5R4 heater is 2A, the 5AR7 is 1.9A and the 5U4 is 3A. I have no idea what a Dynaco transformer can supply.

I believe the 5U4 voltage drop is 44V and the voltage drop for the 5R4 is 60+V so your B+ may go up a bit with a 5U4. The voltage drop for a 5AR7 is about the same as the 5R4 (within a couple volts I think).

It appears the 5AR7 is a better replacement if matching original specs is the goal.

5R4 will work. B+ voltage will be lower, but since it's directly heated, voltage will be high until output tubes warm up. Cathode stripping is not a problem with fixed bias, so filter cap is only worry... they're designed to take SHORT period overvoltage.

Rebiasing required, of course. Output power (especially bass) will be reduced somewhat, not a problem with efficient speakers. In return expect better life from your EL34s..